<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<button onclick="fn()">1</button>
<button onclick="fn2()">2</button>
<button onclick="fn3()">3</button>
</body>
<script>
    function fn() {
        console.log(this);// window
    }
    function fn2(){
        "use strict";
        console.log(this);
    }
    class Box {
        fn3(){
            console.log(this);
        }
    }
    const fn3 = new Box().fn3;
</script>
</html>